1. Field of the Invention
This invention relates to a method of separating and detecting plural light beams propagating in the substantially same direction but which propagate with a small angle therebetween, and more particularly to a method of separating plural light beams, which are substantially parallel to each other, by means of an optical information reading apparatus such as a video disc, a digital audio disc or the like.
2. Description of the Prior Art
Heretofore, there has been known an information reading apparatus wherein a scanning light spot is projected by an objective lens onto information tracks formed spirally or concentrically in, for example, a record medium to read information which had been recorded along the tracks. An example of the record medium having information tracks is the so-called video disc. In this type of the video disc, encoded video and/or audio signals are recorded as optical information such as optical transmissivity, reflection and phase properties. The information recorded on the video disc is read out by focussing a laser beam emitted from a laser light source onto the tracks of the disc through the objective lens as a light spot while rotating the disc at a high speed, and then detecting the transmitted or reflected light beam modulated by the information track. One of important properties of such a record medium is a very high density of recorded information and thus the width of the information track is very narrow and also the space between successive information tracks or the track pitch is very narrow. In order to correctly pick-up the recorded information from tracks having a narrow width and pitch, the laser beam must follow the information tracks. To this end, an error in the position of the light spot with respect to the information track, i.e. a tracking error is detected to produce a tracking error signal and as a result, the light spot is moved in a direction perpendicular to the optical axis of the objective lens and the information track in accordance with the detected tracking error signal. In order to perform such a tracking servo control, there is used a two-beam system or a three-beam system using two or three laser beams substantially in parallel with each other. When such plural laser beams are simultaneously projected onto the record medium, it is necessary that the light beams transmitted through or reflected by the record medium are separated individually and detected by means of plural separate light detectors.
Known methods for separating plural light beams include a method of spatially separating light beams by enlarging a magnification of an optical system and a method of separating light beams which have different wavelengths by a dichroic mirror and the like.
In the former case, however, the light path length must be made large, resulting in the optical system being too large. In the latter case, it is necessary to use light sources having sufficiently different wavelengths, so that it is difficult to use a semiconductor laser or the like.